Thermoplastic elastomers (TPEs) have room-temperature mechanical properties akin to those of vulcanized rubber, but are melt-processible like thermoplastics. The best-known examples are styrenic TPEs, such as polystyrene-polybutadiene-polystyrene triblock copolymers, where the rubbery midblock is the majority. Such all-amorphous block copolymers exhibit many desirable properties, but also have notable shortcomings: poor solvent resistance at room temperature, and very high melt viscosities and elasticities. These deficiencies could potentially be circumvented by replacing the glassy (e.g., polystyrene) “hard” blocks with crystallizable (e.g., polyethylene, E) blocks. By choosing the crystallizable and rubbery blocks to be chemically similar, homogeneous (“disordered”) melts with manageable viscosities and elasticities can be obtained; microphase separation is thus driven by crystallization, rather than by segmental incompatibility. We have compared a range of multiblock polymers, all with poly(ethylene-alt-propylene), EP, as the central block, which forms 80 wt% of the polymer in all cases. The polymers are synthesized by anionic polymerization with chlorosilane coupling, followed by catalytic hydrogenation. Block molecular weights are chosen to ensure the formation of disordered melts. E-EP-E triblocks show good solvent resistance and respectable TPE behavior, but have modest tensile strengths (limited by deformation of the E crystallites), and moduli significantly higher than that of the EP matrix, due to reinforcement by lamellar E domains. Incorporating flanking blocks of glassy poly(vinylcyclohexane), C, in an E-C-EP-C-E pentablock terpolymer, reduces the small-strain modulus twofold, while similarly increasing the tensile strength, by creating a glassy C sheath around the crystallizing E blocks. Recovery following-large strain deformation, and mechanical hysteresis, are also important TPE qualities, which can potentially be improved by the covalent linkage present at the core of a starblock polymer. (E-EP)6 six-arm stars showed 20% higher tensile strengths than E-EP-E linears, while (E-C-EP)6 stars showed 20% lower permanent sets than E-C-EP-C-E linears. All stars showed 30% lower hysteresis than their linear counterparts at moderate strains (up to 200%).

Thermoplastic elastomers from linear and star polymers containing polyethylene blocks

253rd National Meeting (2017)

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